Earthquakes: Causes and Measurements

Earthquakes: Causes and Measurements

Tectonic Relationships

• The phrase "There's just too much friction between us, it's not my fault!" suggests underlying geological tensions.

Where Do Earthquakes Occur?

Geologic Mapping Principles

• Established by Steno in 1669, the key principles include:

  • Law of Original Horizontality: Layers of sediment are originally deposited horizontally under the action of gravity.

  • Law of Superposition: In undeformed stratigraphic sequences, the oldest layers are at the bottom.

  • Law of Original Continuity: Layers of sediment initially extend in all directions until they thin out or encounter a physical barrier.

Types of Earthquake Faults

• The primary fault types associated with earthquakes are:

  • Dip-Slip Faults: Movement occurs vertically along the fault line.

  • Normal Faults: Caused by tension forces; the hanging wall moves down relative to the footwall.

  • Reverse (or Thrust) Faults: Caused by compression forces; the hanging wall moves up relative to the footwall.

  • Strike-Slip Faults: Horizontal movement, where two blocks slide past one another.

  • Right-lateral (dextral): The right side of the fault moves toward the observer.

  • Left-lateral (sinistral): The left side of the fault moves toward the observer.

Characteristics of Faults

• Footwall: The block of rock that lies beneath the fault.

• Hangingwall: The block of rock that lies above the fault.

Distinctions of Fault Types

• Normal Fault

  • Associated with tension forces.

  • Example: Tension in the Earth's crust causing stretching and vertical displacement.

• Thrust Fault

  • Associated with compressional forces.

  • Example: Compression leading to the folding and faulting of rock layers in converging tectonic plates.

• Strike-Slip Fault

  • Caused by shearing forces.

  • Illustration of motion: Right-lateral and left-lateral movements.

  • Example: The San Andreas Fault is a well-known strike-slip fault.

Earthquake Occurrence

• Earthquakes can occur along tectonic plate boundaries but also within plate interiors.

  • Historical Instances:

  • The New Madrid earthquakes (1811-1812) in Missouri.

  • The Charleston earthquake (1886) in South Carolina.

Elastic Rebound Theory

• This explains how energy is released during an earthquake:

  1. Initial Stress: Stress begins with a locked fault, causing elastic deformation.

  2. Continued Stress: Continued stress builds while the deformation occurs.

  3. Fault Slip: When stress exceeds the fault's strength, a rupture begins (the focus).

  4. Stress Release: Post-rupture, stress is released and the rocks rebound to their original, unstressed dimensions.

• Example Scenario: A stone wall built across a right-lateral strike-slip fault shows deformation over time due to stress accumulation and subsequent rupture.

Focus and Epicenter

• Focus: The point within the Earth where an earthquake rupture starts.

• Epicenter: The point on the Earth's surface directly above the focus.

• Fault Scarp: The vertical displacement on the surface along the fault.

• Fault Trace: The horizontal projection of the fault on the surface.

Seismic Waves

• Seismic waves are generated by earthquakes and categorized into two main types:

  1. Body Waves: Travel through the Earth's interior.

  • P (Primary) Waves: Compressional waves that move in the direction of propagation.

  • S (Secondary) Waves: Shear waves that move perpendicular to the direction of propagation.

  1. Surface Waves: Travel along the Earth's surface and cause most damage.

  • Love Waves: Move side-to-side.

  • Rayleigh Waves: Move in an elliptical motion, like ocean waves.

Modern Seismographs

• A modern seismograph consists of:

  • A mass that remains still due to inertia, while the ground moves underneath.

  • A pen that records the ground motion on a rotating drum.

Locating the Epicenter

• Determining the distance from an epicenter requires data from at least three seismic stations.

• The difference in arrival times of P and S waves at a recording station illustrates a function of the distance from the epicenter.

Magnitude vs Intensity Measurements

• Magnitude: A scientific measurement of the energy released at the source of the earthquake, traditionally measured by the Richter scale.

• Intensity: A subjective measure of how the earthquake affects people and buildings, often described by the Modified Mercalli Intensity Scale.

Richter Magnitude Scale

• Based on the amplitude of the seismic waves measured by a seismograph.

• Key points:

  • Each increase of one unit on the scale represents a tenfold increase in measured amplitude.

  • Logarithmic base 10 scale: $( ext{Magnitude} = ext{log}_{10}( ext{Amplitude}))$.

• Historical example:

  • Largest Recorded Earthquake: 9.5 (offshore Chile, 1960).

Moment Magnitude Scale

• A more modern and accurate scale that measures the entirety of an earthquake's source.

• Compare to Richter Scale values for significant earthquakes:

  • New Madrid, MO 1812: 8.7 (Moment Magnitude 8.1)

  • San Francisco, CA 1906: 8.3 (Moment Magnitude 7.7)

  • Prince William, AK 1964: 8.4 (Moment Magnitude 9.2)

Modified Mercalli Intensity Scale

• Developed by Giuseppe Mercalli, this scale categorizes earthquakes based on observed effects and human perception.

• Intensity Levels:

  • I (Not felt): Only a few under favorable conditions.

  • II (Felt by a few): Especially on upper floors of buildings.

  • III (Noticeable): Indoor persons feel it, vibrations similar to a passing truck.

  • IV (Felt indoors by many): Windows, doors disturbed, walls cracking.

  • V (Awakening many): Unstable objects overturned, minor damage.

  • VI (Frightening to many): Heavy furniture moved, slight damage.

  • VII (Negligible damage to good structures): Some damage to poor structures; chimneys fallen.

  • VIII (Considerable damage): Homes seriously affected.

  • IX (Great damage): Well-designed structures thrown out of plumb, buildings shifted off foundations.

  • X (Some structures destroyed): Few remain standing.

  • XI (Total damage): Distorted lines of sight.

  • XII (Complete destruction): Objects thrown in the air.

Future Earthquake Hazard

• Areas of highest expected loss in the USA include California and Washington.

• The lowest expected losses are projected for states like Hawaii and North Dakota.

Recent Earthquake Information

• Current earthquake activity can be monitored via resources such as the USGS earthquake map: https://earthquake.usgs.gov/earthquakes/map/